The transit time of carriers through an active region is an important consideration in maximizing the speed of a semiconductor device. For example, reducing the transit time through the base region of a bipolar transistor will lead to a higher speed transistor.
Extensive work has been performed within International Business Machines Corp. to minimize base transit time by accelerating carriers through the base region of an NPN transistor. U.S. Pat. No. 4,997,776 to Harame et al. teaches base and collector regions that are epitaxially formed silicon-germanium layers. An impurity profile plot illustrates a suggested grading of germanium content designed to create a drift field to accelerate carriers through a base region. The patent teaches that especially in NPN transistor performance, carriers are accelerated through the base region due to a smaller bandgap created by the grading of germanium. While the smaller bandgap in the base region reduces the barrier for electron injection into that region, the primary benefit is gained by bandgap grading which introduces a drift field, e.g., 15-25 kV/cm, aiding the transport of electrons. Patton et al., IEEE Electron Device Letters, vol. 10 (1989), pages 534-536, report use of a graded Si.sub.1-x Ge.sub.x base material, with x varying from 0 to 0.14 to introduce the drift field that aids the transport of electrons. In IEEE Electron Device Letters, vol. 11 (April, 1990), pages 171-173, Patton et al. report a germanium grading in the base region of a heterojunction bipolar transistor (HBT) to create a drift field of approximately 20 kV/cm, resulting in an intrinsic transit time of only 1.9 ps.
While the research at International Business Machines Corp. has shown some of the advantages of Si.sub.1-x Ge.sub.x in bipolar device design, there are limits to the improvement. Fabrication steps that occur subsequent to the epitaxial formation of the Si.sub.1-x Ge.sub.x base region may adversely affect both the speed and the performance of the HBT. The transistors are particularly susceptible to degradation during subsequent steps which take place at elevated temperatures.
Silicon-germanium heterojunction bipolar transistors have been fabricated by others with concentrations of germanium that exceed those described above. Kamins et al., IEEE Electron Device Letters, vol. 10 (November, 1989), pages 503-505, describe the base region of an HBT with a uniform germanium content of 31 percent. Favorable results were obtained, but the report concludes that further improvement was likely available.
It is an object of the present invention to provide a method for fabricating a high-speed, high-performance semiconductor device.